A down hole bearing assembly is positioned between a down hole drilling motor and a drill bit. Should the drill bit become stuck, reactive torque or “backlash” results in the down hole bearing assembly being rotated in an opposite rotational direction to the normal application of torque by the down hole drilling motor. If backlash is not limited, the momentum of this reversed rotation can sever the mandrel of the down hole bearing assembly. A new down hole bearing assembly is manufactured to close tolerances, with a view to limiting backlash. As wear occurs, shims are inserted during servicing to take up slack and maintain backlash within acceptable limits.